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Physicists at the Hong Kong University of Technology and Science have proved that no machine will ever allow a person to travel through time.

The problem is not that we don?t have the technology yet. Simply put, travelling through time is beyond the limits of the physical laws of the universe.

A group of physicists at the Hong Kong University of Science and Technology (HKUST) led by Prof Shengwang Du reported the direct observation of optical precursor of a single photon and proved that single photons cannot travel faster than the speed of light in vacuum. HKUST's study reaffirms Einstein's theory that nothing travels faster than light and closes a decade-long debate about the speed of a single photon.

Prof Du's study demonstrates that a single photon, the fundamental quanta of light, also obeys the traffic law of the universe just like classical EM waves. Einstein claimed that the speed of light was the traffic law of the universe or in simple language, nothing can travel faster than light. HKUST's team is the first to experimentally show that optical precursors exist at the single-photon level, and that they are the fastest part of the single-photon wave packet even in a so called 'superluminal' medium.

"The results add to our understanding of how a single photon moves. They also confirm the upper bound on how fast information travels with light," said Prof Du. "By showing that single photons cannot travel faster than the speed of light, our results bring a closure to the debate on the true speed of information carried by a single photon. Our findings will also likely have potential applications by giving scientists a better picture on the transmission of quantum information."